The present invention relates to a liquid composition which can be utilized as a base stock blend for high temperature lubricants and to a solid antioxidant additive solubilized for high temperature lubricants. In particular, the composition includes a polyolester, a phosphate ester and a synthetic hydrocarbon and the antioxidant comprises a high molecular weight polyphenol ester.
Recent advances in the field of engines and propulsion systems has led to a need for lubricants that can withstand temperatures exceeding 200.degree. C. for a long period of time and temperatures exceeding 375.degree. C.-400.degree. C. for a short duration (e.g. 10-15 minutes). For example, the advanced engine concepts such as adiabatic or low heat rejection diesel engines have much higher combustion temperatures than conventional engines, resulting in maximum top ring temperatures of 375.degree. C.-400.degree. C. in the piston, ring and liner elements of the engine.
Known lubricants subjected to such a high temperature environment suffer from severe and rapid thermal and oxidative deterioration. Oxidation of a lubricant produces reaction products which eventually form deposits that are detrimental to oil consumption and engine emissions.
Antioxidants typically are added to lubricants to combat oxidation. Conventional mixtures of antioxidants and lubricant base stocks, however, are thermally unstable at the higher temperatures generated in the newer engine and propulsion system designs. That is, conventional antioxidants tend to decompose and/or evaporate at high temperatures and, thus, do not provide any protection of the lubricant. Moreover, if the molecular weight of the antioxidant is increased or its molecular structure changed to improve its thermal stability, the solubility of the antioxidant in conventional mineral oil/polyalphaolefin oil lubricants decreases drastically, often to the extent of becoming insoluble.